Marisa Stole the Precious Thing is a flash animation with original music from doujin circle IOSYS.
The music is about characters from Touhou Project, a series of games (initially) vertical shooter style developed by Team Shanghai Alice, which is a doujin game developer group.
Loose translation: “doujin” is something like amateur or independent.
I played some of the Touhou Project games, but it’s not quite my stile… they are extremely hard, and you have to have lots of patience.

“Marisa Stole the precious thing” is sang by character Alice Margatroid. Marisa is a human magician in the games.. and a little kleptomaniac (she’s always robbing things here and there). So, in this animation, the idea is that she would have robbed something precious to Alice… as you might have noticed after watching the animation, the “thing” is Alice’s heart. XD

I ended up posting the explanation after the videos since it’s kind of a hardcore Otaku stuff and might not be interesting for too many people.

The music became so famous that the group ended up releasing a CD… here’s the cover: